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Thoughts on Throats for the 50 BMG

BY DANIEL LILJA

The throat of a rifle barrel is often an overlooked part of the chamber. The throat, however, is very important, especially as related to accuracy. The bullet engages the rifling in the throat. It's critical that the bullet make the transition from case neck to rifling without losing its axial alignment. It's possible for a bullet that is concentric with the bore while in the case neck to enter the barrel misaligned because of a sloppy throat. An oversized throat can allow the bullet to become distorted or canted as it enters the rifling. As a result the bullet will not be spinning around its center of form and has yaw while still in the barrel. When the bullet leaves the muzzle it will yaw even more, causing greater dispersion on the target than would otherwise be seen.

A true match quality benchrest rifle will have its chamber accurately aligned with the bore centerline. The cartridge case necks are turned to a uniform thickness, fitted to the undersized neck diameter of the chamber. These measures further enhance the bullet's ability to engage the rifling concentric with the barrel. It would be a waste to go to these other accuracy improvements and then have a poor throat design cause problems. It's kind of like restoring an old Winchester Highwall and then painting the metalwork flat black instead of giving it a nice rust blue.

So the importance of the throat can now be clearly seen. The 50 BMG cartridge was not originally designed to be a benchrest-type cartridge, however. It was meant to operate smoothly and flawlessly through a machine gun under less than sunny Saturday afternoon match conditions. Accuracy was not a prime consideration; reliability was. The original specifications for 50 BMG chambers (also known as 50 US Ordinance) calls for a chamber throat diameter of .518" which is .008" over nominal bullet diameter. For consistently accurate shooting, this diameter is too large. Match-type reamers for smaller calibers typically have a throat diameter of .0003" to .0005" over nominal. For a cartridge like the 50, I believe that about .001" over bullet diameter or .5110" is about right for accuracy.

The throat actually consists of three different sections. First and probably of the greatest importance is the already mentioned diameter. How far this removal of the rifling extends into the barrel is referred to as freebore or throat length. This constitutes the second section of the throat. A throat too long for the bullet being used can also cause accuracy problems, especially if it's combined with an oversized diameter. Often the best accuracy with a bullet and barrel combination is realized with the bullet just touching--or almost touching--the rifling.

With a throat too long, the bullet is either seated far out in the neck to reach the end of the throat, or it may have to jump some distance after leaving the case neck. It reminds me of trying to ice fish with a fly rod. The minimum amount of support provided by the case neck will be open to debate, but I like to see a minimum length equal to one half the bullet diameter, not including a boattail. For a fifty caliber bullet this would be about .255" of bearing length. More engagement won't harm accuracy, but it should be remembered that reducing throat lengths and diameters will increase working chamber pressures. Loads that are safe in a conventional throat may cause excessive pressures in an accuracy throat.

The third section of the throat is the actual angle that makes the transition from barrel groove diameter to bore diameter. This is aptly named throat angle and is measured in degrees. The standard throat angle is 2 degrees per side as called out in a 50 BMG chamber print. For most bullets this seems to be an acceptable angle as far as accuracy is concerned. We'll discuss throat angle in more detail, however.

In my opinion the best throat angle, if there is such a thing, is an angle that is tangent to the ogive of the bullet at the points between bore diameter on the ogive and the spot where the ogive and bearing surface meet. (See the drawing illustrating this angle.) The axial distance between these two points on the ogive will change with different ogive radii. Also, there are two different types of ogives, tangent and secant.



If we want to know the actual angle for a particular bullet there are a couple of ways to find it. One method would be to measure the angle on an optical comparetor or coordinate measuring machine. Or the angle could be determined mathematically if some information about the bullet's geometry is known or can be measured.
Along with this article is a listing for a short computer program written in GWBASIC that will compute this angle. Members of the Fifty Caliber Shooter's Association are welcome to copy this program for their own use. The program will ask for the following physical characteristics of the bullet: the diameter of the bullet, the bore diameter of the rifle barrel, the type of ogive (either tangent or secant), the ogive radius in calibers, the ogive length (in the case of a secant ogive), and the meplat (nose) diameter. The computer will then calculate the location of the ogive radius center and in the case of a tangent ogive, its length. Then, using calculus the program will compute the axial distance between the given point of bore diameter on the ogive and the point where the ogive meets the bearing surface. Now, knowing this length and the barrel land height, it's a matter of simple trigonometry to calculate the throat angle.

Having done this for a number of possible bullet designs, a few generalizations can be drawn for 50 caliber bullets. A 6 caliber tangent ogive with a meplat diameter of about .100" will result in a throat angle of close to 1.6 degrees per side. A 7 caliber tangent ogive with the same plat would have an angle of about 1.5 degrees per side. And an 8 caliber tangent-type ogive would be 1.4 degrees on each side. So the standard angle of 2 degrees per side is fairly close to typical tangent ogive-type bullets.

When we look at secant ogive-type bullets, though, the angle can vary substantially. The angle for these ogives depends on ogive length and radius. Often the radius is much flatter for secants. The angle is always steeper for secants as compared to tangent ogives. For secant ogive bullets made like the VLD design, the angle will be fairly constant. Since the new 750 grain Hornady bullet falls into this category, I made some measurements of it and ran the numbers through the program. I found that the angle was 3.56 degrees per side. Wanting to give this bullet a good chance for its best accuracy, I had a reamer ground with a 3.5 degree throat on it. I had the diameter made at .5110" too.

Some may think that specifying a throat angle like this is all for naught since the throat will erode a small amount with each shot. There is no doubt about erosion in 50's being a problem. And I also questioned whether a throat will maintain its angle as it marches, like so many drummer boys, down a bore.

Gunsmith Creighton Audette and I traded some correspondence on this subject recently and he provided me with an interesting article from the March 1956 issue of the AMERICAN RIFLEMAN. This article, written by P. H. Naatanen, a Finnish army officer, described some testing they performed with a 2.087" long, rimmed Russian case firing 200 grain .312" diameter bullets. A total of 13,500 shots were fired through this barrel with measurements of both the throat length, angle, and groove diameter taken every 2000 shots. Represented on a graph, the throat angle seemed to advance at a somewhat uniform shape, and it moved forward at a rate of about .040" per 2000 rounds. The actual groove diameter for most of the barrel's length increased only .0002" in diameter. This was attributed to friction between the bullet and barrel. The writer and his associates felt that throat erosion was caused primarily by hot gases, not friction, and interestingly, that it may have been accelerated by an original throat diameter that was .0028" over bullet diameter. In theory, this allowed excessive amounts of hot gas to escape around the bullet and `burn' the throat even more. The crux of the experiment was that gas erosion and not friction caused the most wear on the throat and barrel. Also, the throat and leade angle seemed to wear at a fairly uniform rate, increasing both in diameter and length but closely maintaining the original angle.

It's obvious that a big 50 BMG round is going to cause more erosion than a diminutive .300 Savage-type case, but it would seem logical that the characteristics of that erosion would be similar. Too, I think that the steels used in rifle barrels today are superior to those of the 1950's, especially modern stainless steel and its ability to resist heat erosion.

I have tried to offer some thoughts on throats for accuracy shooting. With the experimenting that is being done with lathe-turned and ground bullets of various shapes and sizes made from bronze and copper and steel, it's possible that designing a throat to match the bullet shape will enhance accuracy. Fifty caliber shooters have been hindered by a shortage of match quality bullets, and no doubt the experimenting will continue in that search for improved accuracy and higher ballistic coefficients.


10 CLEAR:CLS:PRINT"CODE -THROAT' WRITTEN IN GWBASIC BY DANIEL
LILJA 1990"
20 INPUT"ENTER BULLET DIAMETER (INCHES):";BD
30 INPUT"ENTER MEPLAT DIAMETER (INCHES):";MD
40 INPUT"WHAT TYPE OF OGIVE DOES THIS BULLET HAVE (1=TANGENT
2=SECANT):";QO
50 IF QO=2 THEN INPUT"ENTER THE LENGTH OF THE OGIVE INCHES):";NL
60 INPUT"ENTER THE OGIVE RADIUS (CALIBERS):";RC
70 INPUT"ENTER THE BORE DIAMETER OF THE RIFLE BARREL
(INCHES):";RBD
80 C=(BD-MD)/2
90 R=RC*BD:REM OGIVE RADIUS IN INCHES
100 IF QO=2 THEN GOTO 140
110 REM COMPUTE NOSE LENGTH AND THROAT ANGLE LENGTH FOR
TANGENT OGIVE
120 NL=SQR(R*2*C-C^2):REM FIND NOSE LENGTH
130 CC=(BD-RBD)/2:XX=SQR(R*2*CC-CC^2):GOTO 230:REM CALC AXIAL
DISTANCE BETWEEN 0GIVE TANGENCY POINT WITH BEARING SURFACE
AND BORE DIAMETER ON OGIVE
140 REM COMPUTE X-Y COORD OF OGIVE RADIUS CENTER FOR SECANT
OGIVE
150 HYP=SQR(NL^2+C^2):REM HYPOTENUSE OF TRIANGLE IN OGIVE
160 H=R-.5*SQR(4*R^2-HYP^2):REM HEIGHT OF CIRCULAR SEGMENT
170 A=ATN(C/NL):REM FIND ANGLE OF OGIVE SLOPE
180 HX=H/COS(A):REM HYPOTENUSE OF RIGHT TRIANGLE IN CIRCULAR
SEGMENT
190 X=R*SIN(A):REM FIND AXIAL LENGTH OF X COORD FROM MIDPOINT OF
OGIVE CHORD
200 B=SQR(R^2-X^2):REM FIND LENGTH OF Y COORD
210 YC=(B-HX-(BD+MD)/4):REM FIND Y COORD FOR OGIVE RADIUS CENTER
FROM AXIS
220 XC=BL-(NL/2+X):REM FIND X COORD FOR OGIVE RADIUS CENTER FROM
BASE
230 REM:CALCULATE THROAT ANGLE AND THROAT LENGTH
240 LHT=(BD-RBD)/2:IF QO=l THEN GOTO 350:REM HEIGHT OF LAND
250 BDY=(BD/2)-LHT:REM Y COORD OF BORE DIAMETER
260 XSEC=BL-NL-XC
270 XX=NL/10000+XSEC:REM START FIRST DISC AT ONE HALF THICKNESS
280 DIM Y(5000),XX(5000):REM CALCULUS ROUTINE TO FIND LENGTH OF
THROAT ANGLE FOR SECANT OGIVE
290 FOR D=l TO 5000:REM 5000 DISCS
300 Y(D)=SQR(R^2-XX^2)-YC:REM EQUATION OF CIRCLE = X^2+Y^2=R^2
310 XX=XX+NL/5000
320 IF Y(D)<=BDY THEN GOTO 340
330 NEXT D
340 XX=XX-XSEC
350 ANG=ATN(LHT/XX)*57.3:REM ANGLE OF OGIVE SLOPE AT THROAT
CONTACT POINT IN DEGREES
360 PRINT"":PRINT "THE THROAT ANGLE IS (DEGREES PER SIDE):";:PRINT
USING"##.##"; ANG

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